Genes of the MAT locus control the sexual identity of the yeast Saccharomyces cerevisiae. Yeast of a-type expresses the a-factor and the α-factor receptor Ste2p; α-type yeast expresses α-factor and the a-factor receptor Ste3p. If Ste3p is aberrantly expressed by MATa cells (STE3DAF), the yeast has a mechanism, called receptor inhibition, to block the autocrine mating signal that would be generated. Two groups found α-factor-induced transcription of the gene ASG7 to be essential for receptor inhibition. Roth et al. found ASG7 to be the α-induced gene that was required for receptor inhibition in the STE3DAF yeast and that this inhibition occurred because delivery of Ste3p to the plasma membrane was impaired. If Asg7p was expressed in MATα cells, then, Ste3p signaling was blocked in those cells as well. Kim et al. found that when STE3DAF yeast were stimulated with α-factor, the G-protein β subunit Ste4p was redistributed to internal membranes instead of concentrating in the plasma membrane of the mating projection as happens in MATa yeast. The redistribution of Ste4p was blocked and receptor inhibition did not occur in STE3DAF yeast in which ASG7 was deleted. Furthermore, Asg7p also localized to the internal structures where the Ste4p was found. In its normal genetic context, the ASG7 locus may be important mechanism for shutting off the mating signal after fusion and for allowing the transition from the diploid zygote to the haploid growth. Consistent with this hypothesis, Roth et al. found that if ASG7 is deleted in the MAT a cells and then these are mated with MATα cells, the zygotes exhibit abnormal morphology.
Roth, A.F., Nelson, B., Boone, C., and Davis, N.G. (2000) Asg7p-Ste3p inhibition of pheromone signaling: Regulation of the zygotic transition to vegetative growth. Mol. Cell. Biol. 20: 8815-8825. [Abstract] [Full Text]
Kim, J., Bortz, E., Zhong, H., Leeuw, T., Leberer, E., Vershon, A.K., and Hirsch, J.P. (2000) Localization and signaling of Gβ subunit Ste4p are controlled by a-factor receptor and the a-specific protein Asg7p. Mol. Cell. Biol. 20: 8826-8835. [Abstract] [Full Text]